Various substances have been developed in the past for use as coatings and films to be applied to metal, glass, and ceramic substrates by simple commercial processes such as spraying, dipping, or spinning. Typically, such substances developed for use as coating and films were organic polymer based coatings. While some organic polymer based materials formed some degree of commercial acceptance, many of them had very limited performances in hostile environments, i.e. high temperature, corrosive, or abrasive environments.
In recent years there has been a continuing interest in developing metal-based polymeric coatings, espcially glass-like coatings, as substitutes for organic-polymer based ones. Ideally, such a coating should be refractory, corrosion resistant, electrically insulating, flexible enough to be coated on a wire, and also be applicable at low temperatures compatible with the desired substrate.
Typically, these metal base coatings are made by the so-called sol-gel process. In this process, metal alkoxides of network forming cations, e.g., Si, Al, B, Ti, etc. are used as glass precursors. According to the typical process, the metal alkoxides, such as silcon alkoxide represented by the general formula Si(OR).sub.4 wherein R typically represents an alkyl radical, are dissolved first in an alcohol solvent and then partially hydrolyzed by adding water, usually in a stoichiometric molar excess amount. The partially hydrolyzed alkoxides are then polymerized to form a glass-like network linked by bridging oxygen atoms. Dilute solutions (2-5 equivalent wt % oxides) can be applied to various substrates by dipping, spinning, and drying operations. When applied to a substrate, the partially hydrolyzed glass-like polymers react chemically with the surface and thus cause complete wetting and subsequent adherence to the surface of the substrate. The microporous silica-like polymer film can then be converted to a dense like glass film by relatively low-tempeature heat treatment. In sol-gel applications, however, the formation of voids and cracks is the gel are inevitable due to the drying stresses generated by the evaporation of volatiles or low molecular weight intermediate compounds from the gel micropores.
In the past, efforts to reduce crack formation in the drying process of gels have been made by adding a drying control chemical additive (DCCA) such as formamide to the gel. However, this procedure is impractical because of the long drying time required, e.g. a minimum of 12 hours at a temperature of 60.degree. C.